Going to give my opinion for relative power level of the classes, after some 240 hours, shit it was a long holiday season this year. Note all on nightmare, well except the warrior I was so annoyed after 8 hours wasted I turned that one down to casual:
1. Mage, Nightmare is a bit choose your fights till lvl 6-7 but after that it got easy and even with unoptimized group setting it was a cake walk to get to skyhold After that Super easy, knight enchanter is 2 maybe 3 button mode solo everything X levels above you. Other specs are still doing solid. Bit boring but at least it's just hammer those keys till everything is dead.
2 Rogue Archer: just kite everything, walk around shoot and use 3-4 buttons of abilities to conquer all. In theory this could be the easiest. since I never bothered with specializations and stopped distributing ability points after lvl 11. I mean, was saving those points for specialization, just got bored hunting down stuff and suddenly finished the main quests. Just the need to walk around while killing stuff is tiresome...
a really low 3th place:
Warrior, I just can't do the templar or mage quest at a decent level. it's impossible. After you get specializations it's a lot better, sadly that requires getting to skyhold first... buhaha though luck. BUT it's impossible to do either of those quests on a reasonable grindy char on nightmare. And even on casual it involved a 20 minute fight to kill that demon bastard on the templar path. Okay didn't help that the whole thing bugged out completely and no animations worked anymore (even after re-boots and dozens of re-starts set to lowest options it just bugged out). Still every fight takes soo dreadfully long and it's so boring.
Never tried dual wield rogue guessing it's the draw-backs of rogue (lowest defense of all classes and crappy health compared to warrior) with the shitty game-play of warrior... yeah not even going to try. Warrior is just annoying without going tactical mode in every single fight. the ranged ones never needed to use tactical.
OH and horrible horrible bug or a really crap game design:
If you have 3 people that can dispel magic. It's possible to have rifts bug out.
NEVER dispel all re-spawn demons from rifts, worst case the game gets stuck, best case it ignores all dispels.